This invention relates to a guide roll assembly for guiding solidifying metal strand traveling from a casting mold. It also relates to a method of guiding a solidifying length traveling from a casting mold.
In continuous processes for producing steel and other metals into flat products having thicknesses of from about 250 mm (10 inches) or more down to about 150 mm (6 inch) or less, liquid metal is poured into a casting mold where a thin metal skin forms adjacent the mold wall. A continuous metal strand having a skin surrounding a liquid core then travels from the casting mold and through guide roll assembly while the strand continues to solidify and cool down to lower temperatures suitable for further handling and processing. The traveling solidifying lengths are inherently dimensionally unstable because the cores are in or near their melting ranges and also because metals tend to shrink as they solidify and cool. In addition, metals which undergo solid phase change when cooling may shrink at significantly different rates as they cool. For example, a carbon steel strand may shrink up to 5% or more while traveling from a mold to the next processing step. If the strand shrinks to a thickness less than the roll gaps between the guide rolls, then both sides of a strand may not be sufficiently supported by the guide rolls and the traveling strand may move between the guide rolls. In addition, the various processing operations and handling apparatus also create forces on the strands. For example, there may be a substantial static head of liquid metal in the strand core in typical casting operations which must be opposed by the thin strand walls. If the strand is not sufficiently supported by the guide roll assembly, the strand surface may bulge or develop waves which adversely affect its shape or profile. Also, any uncontrolled movements of the strand which causes the liquid metal core to flow relative to the skin may affect the solidification mechanism and thereby degrade the internal metallurgical quality of the strand.
In addition to product quality, the liquid metal in the core of a solidifying strand length which is not suitably supported may cause internal or external fractures of the strand and shut down the casting line or cause other process difficulties.
In order to support both faces of solidifying strand lengths, the art sometimes during a maintenance cycle adjusts spacers between opposed pairs of guide rolls of a guide roll assembly to present a taper line defined by a series of progressively narrowing guide roll gaps to correct for the estimated shrinkage of the particular metal about to be cast. This usually is very time consuming because there are many frame segments supporting the guide rolls which must be unlocked, shimmed and locked up. Frequently the taper line does not sufficiently correct for shrinkage because the processing conditions change over the course of a casting campaign. Also it has been proposed by several others in the art to improve product quality by adjusting the thicknesses of spacers between frame members of guide roll segments in order to effect a soft reduction of about 0.02% upon a solidifying strand length in the last stage of solidification, which would be very difficult to accurately and consistently locate even with the assistance of a modern computing device. More versatile guide roll assemblies have not been developed because the type of devices which would normally be employed to position the guide rolls require relatively large spaces in which to operate but there is very little space around a guide roll assembly to accommodate such devices and their appurtenant equipment. Also the hostile environment surrounding a metal strand requires increased maintenance of instrumentation and control systems and reduces their reliability.